On 2nd of September, Fujii Masao committed patch:
Support ALTER SYSTEM RESET command.
This patch allows us to execute ALTER SYSTEM RESET command to
remove the configuration entry from postgresql.auto.conf.
Vik Fearing, reviewed by Amit Kapila and me.

Similar question has been asked many times on mailing lists and on IRC. Sometimes it's not mail sending, but file/directory creation, or something else that generally requires some interaction with "world outside of database".
Can it be done? Sure. How, then?